The problem is that the toolset available to someone who is much more intellectually capable is beyond what we can think of.
People are not afraid of AGI because it will behave like a very smart human, people are afraid of AGI because the capability gap will be more like the one we experience between humans and other animals.
The toolset available to humans when dealing with monkeys is literally incomprehensible to the monkeys.
The toolset available to AGI is similarly incomprehensible to humans.
For a nuclear approach, the AI would only have to hijack the least-hackproof of US, Russian, or Chinese arsenals in order to trigger an exchange from all sides. But it would probably opt for a different method that would do less collateral damage to its own resources.
This has been an issue raised since at least the early 2010s if not before, and so (arguably) predates the most recent round of US political polarization. The core arguments are unchanged, but became more urgent as AIs broke through several milestones thought to be decades out, such as defeating top human Go players, cracking the protein folding problem, and passing the Turing test with flying colors.
Is it possible that a new technology could destroy the world? Of course. It could've turned out that nuclear weapons would incinerate the atmosphere upon detonation, as some were worried they would. It could be that the next technological innovation will kill us, there's nothing prevent it in the laws of physics.
AGI is a specific technology we are worried about, because the whole premise is "once we build something that is extremely capable at a variety of things, one thing it will be capable of is destroying the world. Even by accident."
We're already using AI techniques to help with problems in biology like protein folding. Take it a few dozen iterations forward, and these systems will be helping design medicines and vaccines that no human can do by themselves. At that point, what's to stop the system from creating a super-flu that kills everyone? Forget about intent here, how about a bug?
ChatGPT often misunderstands queries, take something like ChatGPT but 100x more capable, do you really think people won't be using it to do things? And given that they will, it could easily have a bug that "oops, incinerates the atmosphere" as a side effect.